1. Field of the Invention
The present invention relates to a vent (ventilation) duct particularly having a cylindrical structure mounted for introducing an outdoor air into an internal combustion engine.
2. Related Art
In a conventional technology, a cylindrical vent (ventilation) duct for introducing outdoor air into an internal combustion engine has been provided, and for such a vent duct, various means and attempts have been performed for the purpose of reducing ventilation air-flow resistance in order for appropriately introducing and guiding the outdoor air flow.
One of such means is provided by Japanese Patent Laid-open Publication No. 2002-156977 (Patent Document 1) as silencing device in which a guide block having a semi-circular cross-section and attaining acoustic function is provided on an inner peripheral surface at an inner downstream side of a bent (curved) portion of a duct.
The silencing device disclosed in the above Patent Document 1 is provided with the guide block having a semi-circular cross-section and having acoustic function is provided on an inner peripheral surface at an inner downstream side of a bent portion of a duct, so that peel-off of fluid at the bent portion can be prevented by the provision of the guide block, as well as reducing noise level.
According to the structure of the conventional vent duct mentioned above, although the guide block has a semi-circular cross section, because of tendency of compact structure of an engine room of an automobile or location of various kinds of auxiliary components or parts, there has been caused constriction or restriction to free layout of the vent duct, and accordingly, it has been required to further reduce ventilation resistance without making enlarge the structure or configuration of the vent duct.